The present invention relates to a method of and apparatus for carrying out chemical-mechanical polishing (xe2x80x9cCMPxe2x80x9d), and, more specifically, to CMP operations performed on semiconductor wafers.
Certain stages of semiconductor device manufacture involve the deposition or other formation on a semiconductor substrate of numbers of alternating layers of various materials, such as conducting, insulating and semiconducting materials. These materials may include insulators, metallic oxides, metals, and glass After being formed on the substrate, the layers are lithographically patterned and may be modified by various chemical and physical processes, for example by chemical or electrical etching following appropriate masking, doping or by ion implantation, to produce on the substratesxe2x80x94now termed wafersxe2x80x94numerous electrical devices and electrical interconnections therebetween.
The wafers are extremely delicate and must be protected against the application of external forces which are sufficiently high to damage the devices and interconnections. Damage to only a few of the devices on a wafer may render the entire wafer unsuitable for intended use.
After various devices and interconnections have been defined on a wafer, it is often necessary to remove part or all of one or more of those remaining portions of the previously deposited layers, leaving the resulting surface defect-free and flat. Such layer removal resulting in a flat, defect-free surface is often termed xe2x80x9cplanarization.xe2x80x9d The deposition of additional layersxe2x80x94and their subsequent planarizationxe2x80x94may follow, as may additional litographic steps. One commonly used planarizing technique is chemical-mechanical polishing.
In a CMP operation, the surface of the wafer from which material is to be removed is held against a polishing pad mounted on a rotatable carrier. Usually, the wafer is held upside-down by wafer carrier via application of a negative pressure to the wafer through the wafer carrier. Either or both of the carriers may be rotated in either direction. A slurry is introduced between the pad and the wafer and is held on the pad. The slurry typically includes an appropriate abrasive suspended in an appropriate chemical etchant. The combined action of the etchant and mechanical abrasion as the wafer and the pad relatively rotate removes a selected amount of material from the wafer. Various methods and apparatus are available to ensure that polishing continues only for as long as necessary and that material that should remain is not removed. See for example U.S. Pat. No. 6,179,688, entitled xe2x80x9cMETHOD AND APPARATUS FOR DETECTING THE ENDPOINT OF A CHEMICAL-MECHANICAL POLISHING OPERATION,xe2x80x9d one of the co-inventors of which is a co-inventor of the present invention. See also commonly assigned U.S. patent application Ser. No. 09/679,836, filed Oct. 5, 2000 in the name of Fu Zhang, and entitled xe2x80x9cCHEMICAL/MECHANICAL POLISHING ENDPOINT DETECTION DEVICE AND METHOD.xe2x80x9d Both of the foregoing documents are incorporated by reference hereinto.
It has been observed that during CMP vibrations often occur in the moving carrier-wafer-slurry-pad-carrier system and in associated elements of CMP polishing apparatus. Of course, vibration is common in equipment having rotating parts. However, given the non-robust nature of the wafers, damage thereto may well follow such vibration. Vibrations in the CMP apparatus may also damage various tool sensors, such as those described in the foregoing ""688 patent, and may even cause a wafer to separate from its carrier, resulting in catastrophic destruction of the wafer when it strikes another object.
It has also been observed that the above-described vibrations are difficult, if not impossible, to predict before initiating a CMP process; a non-vibrating CMP process may suddenly experience extreme vibrations for seemingly unknown reasons. Vibrations occurring during CMP may be mild or quite severe in intensity, but they are not usually monitored in present day CMP operations. It is known to attach accelerometers to the frame of machinery performing CMP, but this expedient is usually a response to past severe vibrations that have already damaged one or more wafers.
Accordingly, there exists a need for methods and apparatus for detecting vibrations during CMP operations and for analyzing these vibrations to eliminate them from both an on-going CMP operation and from future CMP operations. The present invention is intended to fill this need.
With the above and other desiderata in mind, the present invention comprises a method of and apparatus for performing CMP of a surface of a semiconductor wafer. Typically, in performing CMP, a wafer on a rotatable carrier is held against a polishing media-carrier, which may be a pad held on a rotatable carrier. Normal force is applied to the wafer and/or the pad to maintain the wafer surface in engagement with the pad. Rotative forces may be applied to the wafer carrier and/or the pad carrier to effect relative rotation between the engaged wafer surface and pad.
Vibrations may occur during the performance of CMP. These vibrations may be deleterious to the CMP equipment and, more to the point, to the wafer. The vibrations are manifested as variations in the normal and rotational forces.
To minimize or eliminate the vibrations after the initiation of a CMP operation, one or more of the applied forcesxe2x80x94that is, the normal force effecting engagement between the wafer surface and the pad and/or the rotational forces applied to the wafer carrier and/or the pad carrierxe2x80x94are continuously measured per unit time. The standard deviation of each measured force is calculated by using the measured values of force(s) and CMP is adjusted in response to the magnitude of the standard deviation to minimize the standard deviation.
Two species of the foregoing are contemplated. In one, a CMP operation is carried out and the standard deviation of all of the measured forces is calculated. This standard deviation is analyzed to determine the likelihood that the wafer just planarized may be damaged and to adjust the CMP for the next and subsequent polishings. This process may be iterated for subsequent polishing operations.
The other species involves adjusting an on-going CMP operation in real time in response to the standard force deviation calculations.
In both species, the measured force(s) may be the normal force, one or more of the rotational forces, or both the normal force and one or more of the rotational forces.
One embodiment of the second species cumulates force measurements and recalculates a new standard deviation following each measurement. Preferably, standard deviation calculation based on these measurements is not begun until CMP proceeds for a time sufficient to cumulate enough force measurements to calculate a meaningful standard deviation. The cumulation may persist for a time period called a xe2x80x9cfixed time window.xe2x80x9d After a first standard deviation is calculated, subsequent standard deviations are calculated for measurements taken during the fixed time window, that is, for the same number of measurements. Thus, it is preferred that subsequently used fixed time windows are all the same length. The fixed time windows may either not overlap or may overlap a selected amount.
The time window may also be a xe2x80x9cdynamic time window,xe2x80x9d that is an ever expanding time window the use of which results in each standard deviation calculation, following each new force measurement, encompassing all of the previous force measurements.
Other embodiments contemplate terminating the CMP operation if a measured force exhibits a predetermined characteristic. Here, a running average of the measurements of one or more of the forces is calculated per unit time. The extant running average is subtracted from each measured force value and the CMP operation is terminated if any resulting difference exceeds a predetermined limit.